Device and method for controlling the pressure in an inflatable cuff of a blood pressure manometer

ABSTRACT

The invention relates to a method and a device for regulating the pressure in at least one inflatable cuff, preferably a finger cuff ( 6 ), of a blood pressure manometer comprising a plethysmographic sensor device ( 8, 9 ) for detecting a plethysmographic signal PG and a pressure sensor ( 7 ) for detecting a cuff pressure signal BP. According to the invention, two control loops ( 1, 2 ) acting on a differential amplifier ( 10 ) are used to independently regulate different operating parameters, the first, inner control loop ( 1 ) using the cuff pressure signal BP as the first regulating variable, and the second, outer control loop ( 2 ) comprising a regulating device ( 12 ), preferably a PID regulator, which generates a nominal value SW as a second regulating variable from the plethysmographic signal PG. The differential amplifier ( 10 ) is connected, on the output side, to at least one valve connected to a pressure source ( 4 ), preferably a proportional valve ( 3; 25, 27 ), for regulating the pressure in the cuff ( 6 ). Additional outer control loops ( 16  to  21 ) can be used to respectively set a parameter of the device to a determined nominal value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device and a method for controlling thepressure in at least one inflatable cuff, preferably a finger cuff, of ablood pressure measuring apparatus provided with a plethysmographicsensoring device, where a plethysmographic signal PG and a cuff pressuresignal BP are obtained.

2. The Prior Art

The continuous monitoring of blood pressure in an artery in anon-invasive way has for years been of interest to scientists andresearchers. As early as in 1942 R. Wagner in Munich presented amechanical system which was designed to measure arterial pressure of thearteria radialis by means of the so-called “vascular unloadingtechnique”, which is also known as the principle of the relaxed arterialwall (Wagner R. “Methodik und Ergebnisse fortlaufenderBlutdruckschreibung am Menschen”, Leipzig, Georg Thieme Verlag, 1942;Wagner R. et al. “Vereinfachtes Verfahren zur fortlaufenden Aufschriftdes Blutdruckes beim Menschen”, Zschr. Biol. 112, 1960). The method fornon-invasive blood pressure determination presented by J. Penaz inDresden in 1973 (Digest of the 10^(th) Inter-national Conference onMedical and Biological Engineering 1973 Dresden), also employs thevascular unloading technique. Due to this technique it was possible forthe first time to continuously record—albeit only for a short period—theintra-arterial blood pressure with the use of an electro-pneumaticcontrol loop. In this method light is shone through a finger andpressure is applied to the finger via a servo-control system in such away that the originally pulsating flow registered by the transmittedlight is kept constant.

The method in principle is based on the following control loop: a limbor part of the human body containing an artery, such as a finger, carpusor the temple, is shone through by light from a light source. The lightwhich passes through the limb (e.g. finger) or is reflected by a bonecontained in the limb or body part (e.g. carpus, temple) is detected bya suitable light detector and provides an inverse measure for the volumeof blood in the extremity (plethysmographic signal PG). The more bloodthere is in the extremity, the more light is absorbed and the smaller isthe plethysmographic signal PG. The mean value of PG is suppressed by adifference amplifier, and the resulting signal PG is fed to a controllerunit. In the method of Penaz this controller has aproportional-integral-differential (PID) characteristic. The controlsignal generated by the PID-controller is amplified and added to aconstant set-point value (SP) and fed to a servo-or proportional-valve,which generates pressure in a cuff, which in turn acts on the extremityshone through by the light. The control system is such that theplethysmographic signal PG is kept constant over time by means of thepressure applied. When the heart pumps more blood into the extremityduring systole and the PG signal exhibits a tendency to decrease, thePID-controller increases its control signal, and pressure in the cuffrises until the surplus blood is pushed out of the extremity and the PGsignal reverts to its previous value. Conversely, when the blood flowinto the extremity decreases during diastole with the heart in itsfill-up phase, which would lead to a rise in the PG signal, the controlsignal of the PID controller decreases and causes the pressure appliedto the finger to drop. Thus the plethysmographic signal is keptconstant. By this control system, which keeps the PG signal and therebythe volume of blood in the extremity constant over time, the pressuredifference (transmural pressure) between the intra-arterial pressure andthe applied external pressure is zero. Thus the externally appliedpressure, i.e. the cuff pressure BP, equals the intra-arterial pressurein the extremity. This permits indirect measurement of the bloodpressure by means of a pressure sensor or manometer.

The above description of the Penaz principle assumes the control systemto be in “closed loop” operation. The system may also operate under“open loop” conditions, with the control signal of the PID controllernot added to the set-point value SP. The pressure in the cuff now doesnot depend on the plethysmographic signal PG and is solely determined bythe set-point value SP. According to Penaz, SP corresponds to the meanarterial pressure in the extremity and is characterised by maximalpulsation of the PG value.

This photo-plethysmographic method has been used in a number of furtherprocedures and devices for the measurement of blood pressure. EP 0 537383 A1 shows an inflatable finger cuff for non-invasive continuousmonitoring of blood pressure. The inflatable cylindrical space of thecuff is pneumatically connected to a fluid source. An infrared lightsource and a detector are positioned on opposite sides of the finger ina rigid cylinder. There is furthermore provided a valve for filling thecylinder with a gas. Electrical leads for the light source and thedetector are passed through the cylinder wall. U.S. Pat. Nos. 4,510,940A and 4,539,997 A also show devices and methods for continuous,non-invasive blood pressure measurement. A fluid-filled cuff, a lightsource, a light detector and a difference pressure amplifier areprovided. Similar devices for blood pressure measurement are known fromU.S. Pat. No. 4,406,289 A.

From WO 00/59369, whose subject is a continuous, non-invasive bloodpressure measuring apparatus, an improvement of the proportional valveor rather the pressure generating system is known, together withvariants of pressure cuffs for diverse extremities.

All known methods and devices—while partly proposing substantialimprovements concerning cuff, proportional valve, determination of theset-point SP, etc.—have one thing in common with the original measuringprinciple of Penaz: a relatively simple control system operated in“closed loop” mode with a controller, e.g. a PID controller. The controlsystem described by Penaz presents a challenge to automatic controlengineering. The following independent systems each with specificdisturbance variables are part of the control system:

-   -   Pressure generation with pressure source (pump) and proportional        valve—pump pressure and valve leakage may vary.    -   Pressure chamber, cuff and pressure transmission to the arterial        blood vessel via the tissue of the extremity.    -   Pulsating fluctuations of blood flow due to the action of the        heart—this is the intended disturbance variable, which is to be        compensated by the cuff pressure in accordance with Penaz's        principle.    -   If the extremity used is the finger, the arterial blood vessel        is a so-called resistance vessel. This means that the diameter        of the artery—and thus the blood volume—may be increased        (vasodilatation) or decreased (vaso-constriction) by the        autonomous nervous system via the smooth muscles of the vessel        wall.    -   Light source and light detection system. Disturbance variables        here are manufacturing tolerances of the parts used and above        all the influence of ambient light on the plethysmographic        signal PG.    -   Mean value suppression of the PG signal.    -   Further disturbances due to fluctuations in the parts used, or        due to electrical or mechanical influences.

These factors almost preclude the possibility of continuous bloodpressure measurement according to the Penaz principle over a long periodof time, even if the set-point SP is optimally determined under openloop operation.

In U.S. Pat. No. 4,510,940 A an effort was made to cope with thesedisadvantages. A method for long-time blood pressure measurement isdescribed, in which closed loop operation is interrupted periodicallyand SP is newly determined under open loop operation. This method is acompromise and has the disadvantage that blood pressure fluctuationsoccurring during the periodic search for optimum SP are not detected.

It is the aim of the present invention to propose, based on theinitially described methods and devices, an improved control procedureand a corresponding apparatus for blood pressure measurementimplementing the procedure, in which a plethysmographic signal PG and acuff pressure signal BP are obtained. In particular, long-time indirectmeasurement of continuous blood pressure is to be guaranteed.

The invention achieves this aim by

-   -   a) using the cuff pressure signal BP in a first, inner control        loop as control variable and feeding it as a first input signal        into a difference amplifier,    -   b) feeding the plethysmographic signal PG, with mean value PG        suppressed, into a controller, preferably a PID controller, in a        second, outer control loop, adding a set-point signal SP and        generating a target signal SW, which is fed as a second input        signal into the difference amplifier, and    -   c) by using the output signal AS of the difference amplifier to        control at least one valve connected to a pressure source, i.e.        preferably a proportional valve, which in turn regulates the        pressure in the cuff.

A device for controlling the pressure in at least one inflatable cuff,preferably a finger cuff, of a blood pressure measuring apparatus, whichhas a plethysmographic sensor device for obtaining a plethysmographicsignal PG and a pressure sensor for obtaining a cuff pressure signal BP,is characterised in that two control loops acting on a differenceamplifier are provided, where the first, inner control loop uses thecuff pressure signal BP as a first control variable and where thesecond, outer control loop is provided with a controller, preferably aPID-controller, which generates a target variable SW from theplethysmographic signal PG as a second control variable, and where theoutput of the difference amplifier controls at least one valve which isconnected to a pressure source, i.e. preferably a proportional valve,thereby regulating the pressure in the cuff. The second control loop isprovided with a difference amplifier of known design, which subtractsthe plethysmographic signal PG from its mean value PG, and with asummation unit (13) adding a set-point signal SP.

The present invention describes a novel control procedure which willpermit long-time indirect measurement of continuous blood pressure. Thecontrol procedure can be realised either as electronic circuitry or itmay be implemented on a computer having program and data-storagecapabilities. Peripheral control loops can preferentially be implementedon a computer in program form, while faster, inner control loopscontaining the drivers for the pressure generation system or for thelight-generation and light-detection systems, are preferably realised aselectronic circuits. A precise distinction between software andelectronic circuitry will not be required in the context of theinvention.

The basic principle of the proposed control procedure consists inproviding specific control loops, which are preferably concentric, forprecisely defined temporal properties and parameters of the wholecontrol system (fast pressure build-up and decrease, compensation oftransmural pressure over a single heart cycle, medium-term fluctuations,long-term drifts). Concentric in this case means that the inner controlloop is pertinent to a certain temporal property or parameter of thecontrol system and presents idealised conditions for this temporalproperty to the immediately following outer control loop. Thisimmediately following outer loop may act as an inner loop for yetanother outer loop. Preferentially, the inner loops take care of fastcontrol tasks, while the outer loops are responsible for the long-termstability of the overall control system. Furthermore, there may beprovided dedicated control loops for certain specific quantities (e.g.cuff pressure, light detection system, mean value suppression etc.),with control parameters optimised for the respective disturbingvariable. These control loops need not necessarily be concentric in thesense explained above.

The invention will now be explained in more detail with reference to theenclosed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a device according to the invention forcontrolling the pressure in an inflatable cuff of a blood pressuremeasuring apparatus with two control loops,

FIG. 2 shows an extended variant of a device as in FIG. 1 withadditional control loops,

FIG. 3 shows a variant of the invention with separate intake and outletvalves of the inflatable cuff,

FIG. 4 shows circuitry details of a further variant of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a device for controlling pressure in an inflatable fingercuff 6 of a blood pressure measuring apparatus not further shown here.The control system consists of a first, inner control loop 1, whichreceives a target signal SW from a second, outer control loop 2. Theinner control loop 1 comprises a difference amplifier 10 (preferably anoperational amplifier), a proportional valve 3, which receives pressurefrom a pressure source, e.g. a pump 4, a pressure chamber 5 connected tothe cuff 6, and a pressure sensor 7, which converts the pressuregenerated in the pressure chamber 5 or in the cuff 6 into an electricalsignal BP, proportional to the cuff pressure. This electrical signal BP,which represents the intra-arterial pressure curve in the extremity E,is fed into the difference amplifier 10, whereby the first, innercontrol loop is closed. The difference amplifier 10 adjusts its outputvoltage AS in such a way that the voltage between its + input and its −input tends to zero. The difference amplifier 10 adjusts the pressure inthe cuff 6 via the proportional valve 3 in such a way that the voltageproduced by the pressure sensor 7 equals the target value SW.

The second, outer control loop 2 supplies a target value SWcorresponding to the actual pressure at the extremity E (e.g. thefinger), which is necessary to keep the plethysmographic signal PG ofthe plethysmographic sensor device 8,9 constant. The outer control loop2 is now no longer responsible for the specific properties of thepressure generation system consisting of proportional valve 3, pump 4,pressure chamber 5, cuff 6 and the pressure sensor or manometer 7, andcomprises essentially the plethysmographic sensor device, i.e. the lightsource 8 (preferably LEDs) and the light detector 9 (preferably aphotodiode), which determine the volume of blood in the extremity E in aknown way, and a difference amplifier 11, which subtracts the signal PGfrom its mean value PG, and a controller 12, whose control parameters,proportional amplification P, integral amplification I and/ordifferential amplification D can be adjusted. The outer control loop 2is closed via a summation unit 13, which adds the controlled signal tothe preset set-point signal SP and thus supplies the target value SW forthe inner control loop 1.

The simple presentation of the control procedure of the invention givenin FIG. 1 shows the following advantage as compared with the initiallydescribed methods: the inner control loop 1 is optimised for fastpressure changes, while the outer control loop 2 is exclusivelydedicated to the compensation of the plethysmographic signal PG. Theparameters of the individual control loops may thus be optimised fortheir respective tasks. A further difference, which will be elaboratedon later, is the fact that here there is no sharp distinction betweenopen and closed loop operation. In the procedure of the invention openloop operation is given if the loop amplifications P, I and D of thePID-controller 12 are set to zero. For all other settings of theseparameters the control loop 2 is closed.

If long-term changes of the system are to be compensated as well,further control loops, each responsible for a specific task, may beadded. Control loops which compensate certain changes over time (slowmedium-to long-term drifts, but also faster changes in pressure) duringblood pressure recording (closed loop operation) are preferably designedas concentric loops in the sense explained above. Control loops orprocedures which serve to define certain initial conditions (initialvalues for SP, P, I and D, settings of the light detection system, meanvalue suppression of PG, etc.) for the measurement proper, do notnecessarily have to be concentric.

FIG. 2 shows a possible further development of the two control loops 1and 2 based on the basic design shown in FIG. 1. The control loops 16 to21 shown here need not all be present and their sequence may be changed.The schematic presentation of FIG. 2 is supposed to illustrate the basicprinciple of concentric control loops together with the novel featuresas compared with the state of the art. Preferably control loops whichact more frequently on the system as a whole and therefore areresponsible for faster changes, should be designed as inner loops.

The control system with loops 1 and 2 as shown in FIG. 1 is representedin FIG. 2 as the central control system 14 and has input parameters SP,PG, P, I and D, and output parameters BP and PG. Around this centralcontrol system 14 further concentric controllers may be used withoutnecessitating an interruption of measurement in order to determine newinitial conditions under open loop operation, as is the case in U.S.Pat. No. 4,510,940 A. In U.S. Pat. No. 4,539,997 A cited above, a firstand second control loop are also mentioned in a purely formal way, butcontrary to the present invention, what is meant there is only the knownclosed control loop with a PID-controller and an open control loopwithout controller.

The invention proposes that the cuff pressure signal BP is fed to asystole/diastole detector, whose output signal is used as controlvariable in at least one of the control loops 3 to 8 to be describedbelow. The point in time of the systoles or diastoles of blood pressure,which is required for certain control loops, is supplied by thisdetector.

The invention also provides for instance that in a third control loop(mean value correction 16) the mean value PG of the plethysmographicsignal PG is determined and continuously corrected as input signal ofthe second control loop. This controller determines the mean value of PGand, if necessary, adjusts it at the PG input of the second controlloop.

In a further development of the invention the amplification parametersP, I and/or D are optimised in a fourth control loop (amplificationcontrol 17), using the plethysmographic signal PG and the cuff pressuresignal BP, and are continuously corrected as inputs to thePID-controller 12. This control loop is dedicated to monitoring and, ifnecessary, correcting the loop amplifications P, I and D of the secondcontrol loop. To this end the ratio between cuff pressure signal BP andplethysmographic signal PG is continuously monitored and optimised.

In an advantageous variant of the invention the set-point signal SP isreadjusted in a fifth control loop 18 depending on the integral of theplethysmographic signal PG. Here the integral of PG is computed for thetime period between two diastoles. Despite continuous compensation asmall PG-signal is always present as an actuating signal and itsintegral can be computed. Since the control system of the whole systemrequires that the plethysmographic signal PG be kept constant by thepressure applied, the integral of PG over time, i.e. over one heartcycle, must also be constant. If this is not the case control loop 18will act on the system and will change the applied pressure by changingthe set-point SP.

In a further development of the invention the set-point signal SP isreadjusted in a sixth control loop (fuzzy control loop 19) on the basisof derived quantities such as amplitude, mean value, signal waveform,etc. of the plethysmographic signal PG and of the cuff pressure signalBP, using a fuzzy-logic approach. The fuzzy controller 19 compares newheart cycles, demarcated by the systole/diastole detector 15, withpreceding heart cycles. Here both signals BP and PG are monitored. Inaccordance with fuzzy logic the following fuzzy heuristics may forinstance be formulated:

-   -   BP resp. PG has (strongly) increased/decreased, therefore SP is        adjusted upwards/downwards,    -   the ratio of mean pressure to pressure amplitude has become        greater/smaller, therefore SP is adjusted upwards/downwards,    -   the ratio of mean pressure to diastolic pressure has become        greater/smaller, therefore SP is adjusted upwards/downwards.    -   and so on.

According to the invention the set-point signal SP is readjusted in aseventh control loop 20 depending on the waveform of the pulse of thecuff pressure signal BP. The controller 20 “waveform control” alsocompares the new heart cycles demarcated by the systole/diastoledetector 15 with preceding ones, the shape of the cuff pressure signalBP being monitored and compared with the pulse waveform of precedingheart cycles. As is well known, the pulse waveform differs from patientto patient, each patient having a characteristic waveform, just as hehas a characteristic finger print. The pulse waveform depends on thecondition of large and small blood vessels and may change in the courseof the years, but not during the time of a blood pressure measurement.This property may also be used in controlling the cuff pressure. If thepulse waveform changes over time, a physiological vaso-constriction orvaso-dilatation has probably occurred and the set-point, respectivelythe set-point signal SP, must be readjusted.

Finally, neural networks, autoregressive models or self-teaching modelsmay be employed to readjust the set-point signal in an eighth controlloop 21.

The summation unit 22 (FIG. 2) adds the changes of the set-point SPsupplied by the individual control loops 16 to 17 and readjusts thepressure applied to the extremity E via the SP-input of the secondcontrol loop 2 (FIG. 1).

FIG. 3 shows an embodiment of the invention in which the differenceamplifier 10 controls an inlet valve 25 connected to a pressure source 4via a non-inverting amplifier unit 23 and an outlet valve 27 via aninverting amplifier unit 24, the valves preferably being proportionalvalves which are pressure-connected to the inflatable cuff 6. Instead ofone proportional valve 3 (as in FIG. 1) two separate valves—one forpressure increase, one for pressure decrease—are used in this case. Theadvantage of this configuration, though without the use of multiplecontrol loops, is described in WO 00/59369 A2 cited above.

The alternative control loop 1 shown in FIG. 3, which is supplied with atarget value SW, consists of a difference amplifier 10 (preferably anoperational amplifier) acting as controller. The output voltage of thedifference amplifier 10 drives a non-inverting amplifier unit 23 and aninverting amplifier unit 24. Absolute amplification of both units isequal, and thus the output voltage of one unit is equal to the negativevoltage of the other,U ₁ =−U ₂.

The amplifier unit 23 controls a proportional inlet valve 25, which onone side is connected to the pump 4 via a pressure compensation vessel26. This inlet valve 25 controls the entry pressure into a pressurechamber 5, which is pressure-connected to the cuff 6. The amplifier unit24 controls a proportional outlet valve 27, which on one side isconnected to the pressure chamber 5. This outlet valve 27 controls theoutlet pressure of the pressure chamber 5 against normal atmosphericpressure. If the output voltage of the difference amplifier 10increases, the output voltage of the non-inverting amplifier unit 23will increase and the output voltage of the inverting amplifier unit 24will decrease by the same amount. Thus the inlet valve 25 will be openedand the outlet valve 27 will be closed to the same degree. Pressure inthe cuff 6 will rise rapidly. If the output voltage of the differenceamplifier 10 decreases the opposite will happen. The outlet valve 27will be opened via the inverting amplifier unit 24 and the inlet valve25 will be closed via the non-inverting amplifier unit 23 by the sameamount, causing a pressure decrease in the pressure chamber 5 and in thecuff 6. A pressure sensor or manometer 7 converts the pressure generatedin the presssure chamber 5 into the cuff pressure signal BP, which isproportional to the pressure and is fed into the difference amplifier10, whereby the first control loop is closed. Ideally the differenceamplifier 10 adjusts its output voltage in such a way that the voltagebetween its +input and its −input tends to zero. The differenceamplifier 10 steers the inlet valve 25 and the outlet valve 27 via theamplifier units 23 and 24 in such a way that the voltage generated bythe pressure sensor 7 equals the target value SW.

The circuit shown in FIG. 3 advantageously works also with non-linearvalves 25 and 27 and even with fast digital on/off switching valves.According to the invention the difference amplifier 10 may be designedas a comparator, which actuates at least one digital switching valveregulating the pressure in the cuff 6. The comparator in this case actsas an operational amplifier with maximum amplification (withoutamplification feedback). The comparator 10 compares SW and BP. If BP isless than SW the output voltage is approximately equal to the positiveoperating voltage, and the inlet valve 25 is completely opened via theamplifier unit 23 and the outlet valve 27 is completely closed via theamplifier unit 24. The pressure generated in the pressure chamber 5increases until BP is greater than SW. The output voltage of thedifference amplifier (comparator) 10 is then approximately equal to thenegative operating voltage and the inlet valve 25 is completely closedwhile the outlet valve 27 is completely open. The pressure generated inthe pressure chamber 5 decreases. If SW and BP are approximately equal arectangular pulse train with a pulse/space ratio of 50% will begenerated at the output of the difference amplifier (comparator) 10.

Information regarding pressure decrease or increase is thus encoded inthe pulse/space ratio of the rectangular pulse train generated at theoutput of the difference amplifier (comparator) 10. A precondition forthe functioning of the system are sufficiently fast switching valves(preferably piezo-valves), which react much faster than the pressurechange in the cuff 6, which is characterised by a certain inertia.

The control loops shown in FIGS. 1 to 3 all are active during continuousblood pressure measurement (closed loop operation). To guarantee correctoperation precisely defined initial conditions must be established—as isthe case for most control loops. The initial conditions are preferablydetermined prior to the measurement proper. It is of no import wetherthis is done in open loop or closed loop operation.

In contrast to the state-of-the-art methods mentioned before, it will beof advantage in the invention if an optimum plethysmographic signal PGis found. The invention therefore provides that the plethysmographicsensor 8, 9 is furnished with a device 28, 40, 41 for the elimination ofstray light, in particular ambient light, from the plethysmographicsignal PG, and further provides circuitry 33 to 38 for controlling thevoltage or current of the light source 8 of the plethysmographic sensor.

FIG. 4 shows a possible variant of a control system generating anoptimal PG-signal. That part of the system which eliminates ambientlight, is a concentric control loop, the part which sets the optimalLED-current (light source 8), however, is not, since it defines aninitial value.

The light source 8, a LED, is controlled by a timer unit 28, whichgenerates three synchronous rectangular pulse trains. The signal “LED”29 pulsatingly activates the LED 8. When the signal “LED” 29 is at HIGHthe LED 8 is turned on. The pulse/space ratio of 50% shown in FIG. 4 isnot essential, and other ratios are possible. The timer 28 furthermoregenerates a signal “SH_(light)” 30 (Sample & Hold), which is HIGHimmediately before the LED 8 is turned off. The signal “SH_(dark)” 31also generated by the timer is HIGH immediately before the LED 8 isturned on.

The signal 29 turns on the LED 8 by means of switch 32. The unitLED-control 33 can change the current of LED 8 and thus the lightintensity. Switches 34 and 35 can be used to activate the shuntresistors 37 and 38 parallel to the current limiting resistor 36, andthereby the total current through LED 8 can be increased.

The light which is shone through the extremity E, is detected by aphotodiode 9 and amplified by an amplifier 39. The detected light signalwill pulsate as the LED 8 is turned on and off. But a weak light signalwill be detected even if the LED 8 is off, because ambient light willalso pass through the extremity E and generate a signal at thephotodiode 9. In order to avoid transients caused by turning the LED 8on and off, the time immediately prior to the switching of LED 8 willnow be considered. Immediately before LED 8 is turned on—LED 8 is stilldark—the signal generated in the photodiode 9 depends solely on ambientlight. On the other hand, immediately before LED 8 is turned off—withLED 8 still emitting light—the signal generated in the photodiode 9 willdepend on the light of the LED plus ambient light. These points in timeare defined by the timer 28 and its signals “SH_(light)” 30 and“SH_(dark)” 31. The amplified light signal of the photodiode 9 is fed toa sample&hold unit 40 and demodulated by the signals “SH_(light)” 30 and“SH_(dark)” 31. At the output terminals of the sample&hold unit 40 thesignals Light and Dark are generated. If the difference of these twosignals is computed in a difference amplifier 40, there will result alight signal which is only dependent on the light intensity of the LED 8and is free of ambient light effects.

The light signal generated has a predominating DC-component and an addedsmaller signal PG, which is the desired plethysmographic signalcorresponding to the pulsating changes of blood volume due to the actionof the heart. The DC-component or rather the mean value of the PG-signalPG is not relevant for the blood pressure measurement proper, but is adisturbance and must therefore be suppressed. PG is however dependent onthe extremity E chosen for measurement and differs greatly betweenpatients. According to the invention the control system therefore isfurnished with a device 42 to 47 for computing a starting value for themean value PG of the plethysmographic signal. Mean value correction musttake place reliably before each measurement run and is performed in thefollowing way:

Mean value correction proper is carried out by the difference amplifier11, which is pre-supplied with a certain mean value PG as an initialvalue. The difference amplifier 11 generates the PG-signal bysubtracting the light signal from which ambient light has already beenremoved, from the pre-supplied mean value PG. The difference amplifier11 not only carries out mean value correction, but also amplifies andinverts the PG-signal, which is then fed into a comparator circuit. Thecomparator circuit consists of an upper comparator 42, a lowercomparator 43 and a voltage divider with resistors 44, 45, 46, whichdefines threshold values. If the PG-signal generated by the differenceamplifier 11 exceeds the threshold value of the upper comparator 42,this indicates that the pre-supplied mean value PG has been chosen toohigh. This is communicated to the unit PG Control 47, which decreasesthe mean value PG until the PG-signal is below the threshold value ofthe upper comparator 42. If, on the other hand, the PG-signal generatedby the difference amplifier 11 is smaller than the threshold value ofthe lower comparator 43, the pre-supplied mean value is too low. In thiscase the unit PG Control 47 will increase the mean value PG until thePG-signal lies above the threshold of the lower comparator 43.

The control system shown in FIG. 4 also contains a Peak Detector 48 fordetermining the maximum amplitude of the PG-signal, which occurs, as iswell known, when the pressure BP in the cuff 6 is approximately equal tothe mean blood pressure. Thus the Peak Detector 48 may be used to findthe mean blood pressure. To this end the pressure BP in the cuff 6 isvaried until the maximum amplitude of the PG-signal occurs. The maximumamplitude of the PG-signal obtained is now assessed, since it alsodepends on the properties of the extremity E of each patient. If theamplitude is too small, the unit LED Control 33 is instructed toincrease the current and thereby the light intensity of the LED 8.Conversely, if the maximum amplitude of the PG-signal is too large, theLED Control unit 33 is instructed to reduce the current through the LED8.

When the cuff pressure BP at which the amplitude of the PG-signal is atits maximum, has been found, the optimum current for LED 8 can bedetermined—as described above—and the interfering mean value PG may beeliminated from the PG-signal in an optimal way by means of thecomparator circuit 42-47 described above. The obtained pressure BP alsocorresponds to the optimum initial value of the set-point SP, because avariation of the pressure BP is mainly effected by a change in SP, withthe control amplifications P, I and D set to zero.

Finally the invention proposes a means for computing an initial valuefor the set-point signal or the set-point SP.

To determine an optimum set-point value SP the following procedure ispreferably used: After

-   1. the pressure BP in the cuff 6 has been changed by a variation of    SP in such a way that the Peak Detector 48 has found the maximum    amplitude of the PG-signal, and-   2. the optimum current for LED 8 has been found, and-   3. the interfering mean value PG has been optimally eliminated from    the PG-signal,

the amplification control unit 17 (FIG. 2) computes P, I and D from themaximum amplitude of the PG-signal and closes control loop 2. Pressurein the cuff 6 starts to pulsate and in accordance with the imposedcontrol condition the PG-signal is held constant. Now SP is againvaried. The systole/diastole detector 15 demarcates each heart cycle andassociates it with the corresponding SP, so that it may be assessed fordetermination of the optimum SP. Preferably a typical heart cycle foreach different SP is used for assessment. The following criteria may beused for assessment: the amplitude of BP, the ratio of mean pressure topressure amplitude, the ratio of mean pressure to diastolic pressure,pressure rise or decay, temporal relationships etc. In accordance withthe fuzzy logic in control loop 19 the following fuzzy criteria can beformulated and the heart beats or heart cycles may be assessedaccordingly:

-   -   the BP amplitude of the heart cycle considered is in the range        of maximum BP amplitudes    -   the ratio of mean pressure to pressure amplitude lies in the        physiological range    -   the ratio of mean pressure to diastolic pressure lies in the        physiological range    -   pressure rise and pressure decay are in the physiological range    -   temporal relationships are in the physiological range    -   and so on.

In accordance with these fuzzy criteria each heart cycle may beassessed, for instance by using a simple scoring system. The heart cyclewith the best score has the optimum SP, ties are resolved by using themean value of the optimum SPs. In this way the optimum starting value ofthe set-point, or the set-point signal SP, can be found and communicatedto all concentric control loops. The measurement proper may now begin,since all initial values of the control loop, SP, PG, P, I, D and theoptimum LED current, have been determined. These values are nowmonitored and, if necessary, readjusted by all concentric controllersand long-time continuous measurement of blood pressure is made possible.

The invention claimed is:
 1. A method for controlling the pressure in atleast one inflatable cuff of a blood pressure measuring apparatus inclosed-loop operation, where the pressure on the cuff equals thearterial pressure, with a plethysmographic sensor device, whereby aplethysmographic signal PG and a cuff pressure signal BP are obtained,comprising the following steps: a) in a first concentric inner controlloop the cuff pressure signal BP is used as control variable and is fedinto a difference amplifier as a first input signal, b) in a secondconcentric outer control loop, which is simultaneously active with thefirst concentric inner control loop for closed-loop operation, theplethysmographic signal PG, with its mean value PG suppressed, is fedinto a controller and is added to a set-point signal SP, and a targetsignal SW is generated, which is fed into said difference amplifier as asecond input signal, c) an output signal AS of the difference amplifieris used to control at least one valve connected to a pressure source,which in turn regulates the pressure in the cuff, and d) nore-adjustments of set-point signal SP during open-loop operation withthe help of a state-switch and timing-circuits.
 2. The method accordingto claim 1, wherein the mean value PG of the plethysmographic signal PGis determined in a third concentric control loop, which issimultaneously active with the first concentric inner control loopduring closed-loop operation, and continuously corrected as input signalof the second control loop.
 3. The method according to claim 1, whereinthe amplification parameters P, I and/or D are optimized in a fourthconcentric control loop, which is simultaneously active with the firstconcentric inner control loop during closed-loop operation, and by meansof the plethysmographic signal PG and the cuff pressure signal BP, andare continuously corrected as inputs to the controller.
 4. The methodaccording to claim 1, wherein in a fifth concentric control loop, whichis simultaneously active with the first concentric inner control loopduring closed-loop operation, the set-point signal SP is readjusted,depending on the integral of the plethysmographic signal PG.
 5. Themethod according to claim 1, wherein in a sixth concentric control loop,which is simultaneously active with the first concentric inner controlloop during closed-loop operation, the set-point signal SP is readjustedon the basis of derived quantities, such as amplitude, mean value orwave form of the plethysmographic signal PG and the cuff pressure signalBP, using a fuzzy-logic-approach.
 6. The method according to claim 1,wherein in a seventh concentric control loop, which is simultaneouslyactive with the first concentric inner control loop during closed-loopoperation, the set-point signal SP is readjusted in dependence of thepulse waveform of the cuff pressure signal BP.
 7. The method accordingto claim 1, wherein in an eighth concentric control loop, which issimultaneously active with the first concentric inner control loopduring closed-loop operation, the set-point signal SP is readjusted bymeans of neural networks, auto-regressive models or self-learningmodels.
 8. A device for controlling the pressure in at least oneinflatable cuff of a blood pressure measuring apparatus in closed-loopoperation, where the pressure in the cuff equals the arterial pressure,comprising a plethysmographic sensor device for obtaining aplethysmographic signal PG and a pressure sensor for obtaining a cuffpressure signal BP, including two concentric control loops, which aresimultaneously active during closed-loop operation, acting on adifference amplifier, the first concentric inner control loop uses thecuff pressure signal BP as a first control variable and the secondconcentric outer control loop includes a controller which generates atarget variable SW from the plethysmographic signal PG as a secondcontrol variable, and wherein the output of the difference amplifiercontrols at least one valve connected to a pressure source, therebyregulating the pressure in the cuff.
 9. The device according to claim 8,wherein the second concentric control loop, which is simultaneouslyactive during closed-loop operation, is provided with a differenceamplifier which subtracts the plethysmographic signal PG from its meanvalue PG, and with a summation unit adding a set-point signal SP. 10.The device according to claim 9, wherein a device is provided forcomputing an initial value for the mean value of the plethysmographicsignal.
 11. The device according to claim 9, wherein a device isprovided for computing an initial value for the set-point signal SP. 12.The device according to claim 8, wherein said difference amplifiercontrols an inlet valve connected to a pressure source via anon-inverting amplifier unit and an outlet valve via an invertingamplifier unit, said valves being pressure-connected to the inflatablecuff.
 13. The device according to claim 8, wherein said valves beingpressure-connected to the inflatable cuff are designed as proportionalvalves.
 14. The device according to claim 8, wherein said differenceamplifier is designed as a comparator which actuates at least onedigital switching valve for pressure regulation in the cuff.
 15. Thedevice according to claim 8, wherein the plethysmographic sensor isfurnished with a device for the elimination of stray light or ambientlight from the plethysmographic signal PG.
 16. The device according toclaim 8, wherein the light source of the plethysmographic sensor isfurnished with circuitry for controlling its voltage or current.
 17. Thedevice according to claim 8, wherein said at least one inflatable cuffis a finger cuff.
 18. The device according to claim 8, wherein saidcontroller is a proportional-integral-differential PID-controller.